Maraging steels based on 9-12% chromium are materials that are in widespread use in power plant engineering. It is known that adding chromium within the abovementioned range allows not only a good resistance to atmospheric corrosion but also full hardening all the way through thick-walled forgings (as used for example as monoblock rotors or as rotor disks in gas and steam turbines) to be achieved. Tried-and-tested alloys of this type usually contain approximately 0.08 to 0.2% carbon, which in solution allows a hard martensitic structure to be established. A good combination of hot strength and ductility in martensitic steels is made possible to a tempering treatment, in which the precipitation of carbon in the form of carbides with simultaneous annealing of the dislocation substructure leads to the formation of a particle-stabilized subgrain structure. The tempering performance and the resulting properties can be effectively influenced by the choice and quantitative adjustment of special carbide-forming agents, such as for example Mo, W, V, Nb and Ta.
Strengths of over 850 MPa can be established in 9-12% chromium steels by keeping the tempering temperature at a low level, typically in the range from 600 to 650° C. However, the use of low tempering temperatures leads to high transition temperatures from the brittle to ductile state (over 0° C.), with the result that the material has brittle fracture properties at room temperature. Significantly improved ductilities can be achieved if the heat-treated strength is reduced to below 700 MPa. This is achieved by raising the tempering temperature to over 700° C. The use of higher tempering temperatures has the advantage that the microstructural states which are established have long-term stability at elevated temperatures. A typical representative which is in widespread use in steam power plants, in particular as rotor steel, is the DIN steel X20CrMoV12.1.
It is also known that the ductility can be significantly improved at a strength level of 850 MPa by the addition of nickel to the alloy. For example, it is known that by adding approximately 2 to 3% nickel to the alloy, the transition temperature from the brittle to ductile state is still below 0° C. even after a tempering treatment at temperatures of from 600 to 650° C., with the result that overall a significantly improved combination of strength and ductility can be established. Therefore, alloys of this type are in widespread use where significantly higher demands are imposed on both strength and ductility, typically as disk materials for gas turbine rotors. A typical representative of alloys of this type, which is in widespread use in gas turbine engineering, in particular as a material for rotor disks, is the DIN steel X12CrNiMo12.
In recent times, various efforts have been made to improve specific properties of these steels. For example, the publication by Kern et al.: High Temperature Forged Components for Advanced Steam Power Plants, in Materials for Advanced Power Engineering 1998, Proceedings of the 6th Liège Conference, ed. by J. Lecomte-Becker et. al., describes the development of new types of rotor steels for steamturbine applications. In alloys of this type, the Cr, Mo, W contents were optimized further taking account of the parameters of approximately 0.03 to 0.07% N, 0.03 to 0.07% Nb and/or 50 to 100 ppm B, in order to improve the creep resistance and creep rupture strength for applications at 600° C.
On the other hand, specifically for gas turbine applications, efforts have been made to either improve the creep rupture strengths in the range from 450 to 500° C. at a high ductility level or to reduce the susceptibility to embrittlement at temperatures between 425 and 500° C. For example, European Patent application EP 0 931 845 A1 describes a nickel-containing 12% chromium steel, the constitution of which is similar to DIN steel X12CrNiMo12 and in which the level of molybdenum has been reduced compared to the known steel X12CrNiMo12, but a higher tungsten content has been added. DE 198 32 430 A1 discloses a further optimization to a steel of similar type to X12CrNiMo12, designated M152, in which the susceptibility to embrittlement in the temperature range between 425 and 500° C. is restricted by the addition of rare earth elements.
One possible approach for improving the hot strength combined, at the same time, with a high ductility was proposed by the development of steels with high nitrogen contents. EP 0 866 145 A2 describes a new class of martensitic chromium steels with nitrogen contents in the range between 0.12 and 0.25%. In this class of steels, the overall microstructure formation is controlled by the formation of special nitrides, in particular vanadium nitrides, which can be distributed in numerous ways by means of the forging treatment, by austenitization, by a controlled cooling treatment or by a tempering treatment. Whereas the strength is achieved by the hardening action of the nitrides, the patent application in question aims to establish a high ductility by means of the distribution and morphology of the nitrides, but in particular by limiting the grain coarsening during forging and during the solutionizing treatment. In said document, this is achieved by both a high volumetric level and a high particle coarsening resistance of nitrides of low solubility, so that a dense dispersion of nitrides was still able to effectively restrict grain growth even at austenization temperatures of 1150 to 1200° C. The main benefit of the alloys mentioned in EP 0 866 145 A2 is the possibility of optimizing the combination of strength and ductility simply by the distribution and morphology of nitrides, on the basis of a suitable definition of the heat treatment.
However, an optimized nitride state is only one factor in achieving a maximum ductility. A further influencing factor is likely to arise from the effect of dissolved substitution elements, such as nickel and manganese. Within the class of carbon steels, it is known that manganese tends to have an embrittling rather than ductility-enhancing effect. In particular, it causes embrittlement if the alloy is exposed to prolonged annealing at temperatures in the range from 350 to 500° C. It is also known that nickel in carbon steels improves the ductility but also tends to reduce the hot strength at high temperatures. This is related to a reduced carbide stability in nickel-containing steels.
EP 1 158 067 A1 has disclosed a maraging heat-treatment steel having the following chemical composition (details in % by weight): 9 to 12 Cr, 0.001 to 0.25 Mn, 2 to 7 Ni, 0.001 to 8 Co, at least one of W and Mo in total between 0.5 and 4, 0.5 to 0.8, at least one of Nb, Ta, Zr Hf in total between 0.001 to 0.1, 0.001 to 0.05 Ti, 0.001 to 0.15 Si, 0.01 to 0.1 C, 0.12 to 0.18 N, max. 0.025 P, max. 0.015 S, max. 0.01 Al, max. 0.0012 Sb, max. 0.007 Sn, max. 0.012 As, remainder Fe and standard impurities, with the proviso that the vanadium to nitrogen weight ratio V/N is in the range between 3.5 and 4.2. These alloys are distinguished by a very good combination of notched-impact energy at room temperature and hot strength at 550° C., in particular even with relatively high Cr contents. The relatively high N content increases the creep rupture strength. Within the range stipulated, V and N are in virtually stoichiometric proportions. This results in an optimum solubility and resistance to coarsening on the part of the vanadium nitrides. The high solubility is required in order for the maximum possible amount of the precipitation-hardening vanadium nitride to be dissolved, while a high resistance to nitride coarsening is needed in order to be able to achieve a structure which is as fine-grained as possible during the heat treatment described in EP 1 158 067 A1.
It is known that in steels containing approx. 12% chromium and with a high N content, the α′Cr phase disadvantageously precipitates in the temperature range from approximately 425 to 500° C., which leads to embrittlement of the steel. Although these precipitations increase the strength properties, they reduce the ductility, notched impact strength and corrosion resistance. Consequently, steels of this type are of only limited use in compressors or turbines in the power plant sector. The formation of VN in steels of this type also increases the susceptibility to precipitation of the α′Cr phase and therefore the susceptibility to embrittlement within the temperature range mentioned.